I realise this question is a little subjective, but I think there are only a very small number of DBMSs that match the requirement.
I'm looking for some recommendations for a standalone SQL server. Something that can either be bundled on compile, or already have a driver built-in to Windows.
I'd usually use MySQL or MS SQL, depending on environment, but I'd like to "graduate" to SQL for things I'd usually do in XML files for Windows.
The db should be accessible without an internet connection, and preferably be stable and mature. I don't mind what flavour SQL it is. It'd only really be for a single concurrent user at a time, i.e. to store program user data.
I'm looking for something "general-purpose", that I can rapidly integrate with new projects/small tools I make.
From my research, the two I'm coming up with, that still look like they're in development are: the well known SQLite, and also Firebird. What are the pro's and con's of each?
Have I missed any killer standalone SQL servers? I've used Sybase SQL Anywhere in the past, but it has its quirks.
Have you looked at SQL Server Compact Edition? Sounds like it might meet your criteria.